Is it an IRS requirement that pre-tax deductions (like 401K and health) be put on the W-2 (in box 12/14)...or is this optional for the employer?

not optional; employers who provide heath insurance to their employees must report the cost of coverage on their employees' on w2;box 12 contains def compensation that is an arrangement in which a portion of an employee's income is paid out at a later date after which the income was actually earned